Automatic bucket positioning device

ABSTRACT

Hydraulic means operating in conjunction with an hydraulic power unit for automatically stopping in an intermediate position a device whose position is controlled by the hydraulic power unit. A digging bucket is automatically stopped at a digging position intermediate to dumping and breakout positions with respect to a boom arm. A pilot valve connected to an intermediate region of a cylinder of the hydraulic power unit regulates a dump-hold-rack back or roll back valve for the power unit.

States atent 1191 1111 3,792,640

Shore 1 Feb. 19, 1974 AUTOMATIC BUCKET POSITIONING 3,511,133 5/1970 Day 91/388 DEVHCE 3,547,293 12/1970 Borer et a1 214/764 x [75] Inventor: Daniel B. Shore, Niles, Ill.

Primary Examiner-Irwin C. Cohen [73] Ass1gnee: International Harvester Company,

Attorney, Agent, or FirmFloyd B. Harman; John W.

Chicago, Ill. G i es 22 Filed: Aug. 7, 1972 PP 57 ABSTRACT Related US. Application Data I Hydraulic means operating in conjunction with an hy- [62] DlVlSlOI'l of Ser. No. 71,778, Sept. 14, 1970, Pat. No.

draulic power unit for automatically stopping in an intermediate position a device whose position is controlled by the hydraulic power unit. A digging bucket is automatically stopped at a digging position interme- 1 of A, 91/47l;l37/624.27 p

diate region of a cylinder of the hydraulic power unit regulates a dump-hold-rack back or roll back valve for [56] References Cited the power unit. UNITED STATES PATENTS 2,159,879 5/1939 Dewandre 91/388 3 Claims, 2 Drawing Figures J m 56 .55 17 2 751F J r 45 4/ Q Q X (I 47 CARRY PAIENTED FEB] 9 m4 0 4 \I. O H AM C w m 5. A 1 1m 7 a 5 k 9., w W 0 m 3 3 0 MW L 9 8 3 5 a a 3 Q AUTOMATIC BUCKET POSITIONING DEVICE This application is a division of Shore U. S. Pat. Ser. No. 71,778 filed Sept. I4, 1970, now patented Oct. 17, 1972, US. Pat. No. 3,698,583.

The present invention relates to automatic positioning by hydraulic means. More particularly, the invention relates to the use of hydraulic means for automatically placing a device in an intermediate position between extreme positions.

It is highly desirable, for the sake of facilitating operation of a tractor loader, for the bucket device on the front end to assume its intermediate digging position automatically. Such position is intermediate a forwardly tilted, dump position and a racked-back position'of breakout or carry. Taking the position automatically in this way has at times been referred to, even though not necessarily accurately so, as self-leveling.

According to practice in the past, the loader driver has set a bucket positioner valve in an operating position, causing the bucket positioner to rack back the bucket from dump position back to the intermediate digging position. A kickout device has been provided to kickout the valve from operating position to neutral or hold position as soon as, and no later than, the bucket reaches digging position. The practice then has been to interpose a stationary kickout trip. The kickout trip is mechanically tripped by physical contact with a synchronously moving actuator when the moving bucket reaches digging position, and the kickout trip then sets the kickout device in operation to kickout the valve from operating position, causing the bucket to stop and then holding the bucket where it stopped.

It has therefore been the practice to use bucket connected, mechanical actuators for engaging and physically deflecting the trip enough to cause operation. Such actuators and trip are usually made freely accessible from the outside for adjustment, cleaning, and possible resetting such as is usually required in service. Feeler switch contacts or micro-switch contacts are common where the kickout control is primarily electrical, and limit valves are very often used in pneumatic and hydraulic systems. A strict mechanical system for kicking out the valve has been found to act positively but it, too, requires the referred to adjustment, cleaning, and resetting from time to time.

My invention employs a contact-less positioning control having purely an hydraulic connection to kickout the valve of the bucket positioner. The hydraulic principle means freedom from the outside dirt and from loss of mechanical adjustment, because the hydraulics and actuation are inside.

My novel method of automatic kickout involves a contact-free positioning control in an hydraulic power system having: a traveling member which is pressuremovable for performing work; a self-neutralizing valve for the system having an unsatisfied position in which it causes pressure-actuated travel of the pressuremovable traveling member; and a first-signal-locked, second-signal-kicked-out kickout control means for the valve. More specifically, the method according to my invention comprises the steps of: developing a 1st signal in response to pressure-actuation of, and the resulting travel by, the pressure-movable traveling member; applying the 1st signal to the kickout control means as a detaining signal for locking-in the kickout to detain the self-neutralizing valve in unsatified position; sensing and signaling with a second signal when the pressure-movable member is part way in its travel; and applying the second signal to the kickout control means as a valve kickout signal to kickout the self-neutralizing valve and perforce stop travel of the pressure-movable traveling member.

The foregoing operation employs a power cylinder piston having foreshortening movement and extension movement to force the bucket forward or to cause rack back of the bucket, and results in: the piston moving the bucket full forward into extreme dump position when the valve is manually sustained in the extreme depressed position hydraulically directing piston movement of foreshortening; the piston moving the bucket full back into the extreme carry position when the valve is manually sustained in the extreme extended valve position hydraulically directing piston movement of extension; and the piston pivoting and holding the bucket in an intermediate rack back position for digging when the valve is sustained in and then released automatically from the just said extreme extended valve position so as to reach its satisfied position.

The present invention is illustrated and described to show its usage in automatically positioning a digging bucket with respect to a boom arm, but it may have various uses, including bringing the boom arm to some intermediate position with respect to a tractor loader on which the boom arm is mounted.

IN THE DRAWINGS FIG. l is a diagrammatic and sectional view of the hydraulic system of the present invention; and

FIG. 2 is a partial elevational view of a tractor loader to which the present invention is applied.

As shown in FIG. 1, a hydraulic power unit 10 comprises a cylinder 1 1, a piston 12 slidably mounted in the cylinder, and a rod 13 secured to the piston 12 and projecting from one end of the cylinder 11. A control valve 14, which comprises a stationary housing 15 and a movable spool 16 slidably mounted in the housing 15 is connected with a pump 17 by a line 18, by a line 19 with drain indicated at 20, and by lines 21 and 22 with openings 23 and 24, respectively, in the cylinder 1 1 adjacent its opposite ends. When the spool 16 is in the central position shown, the lines 18 and 19 are connected with one another so that the pump 17 takes hydraulic fluid from drain indicated at 24a and returns the fluid directly by way of lines 18 and 19 to drain at 20. In the central or hold position of spool 16, lines 21 and 22 are blocked so that the position of the piston 12 in the cylinder 11 is fixed. When the spool 16 of the control valve 14 is displaced to the right as viewed in FIG. 1 from the position shown in this figure, the piston 12 moves to the right as viewed in FIG. I, because the left end of the cylinder 11 is connected with the pump 17 by way of lines 18 and 20, and the right end of the cylinder with drain at 20 by way of lines 22 and 19. When the spool 16 is displaced to the left as viewed in FIG. 1 from the position shown, the lines 21 and 22 are reversed in their connections to pump line 18 and drain line 19, and so the piston moves to the left as viewed in FIG. 1.

The spool 16 of the control valve 14 is yieldingly held in the central position shown in FIG. 1 by means of a coil spring 25 and two cup-like members 26 and 27. The ends of the coil spring 25 act against outwardly directed flanges on the members 26 and 27, urging them against shoulders 28 and 29, respectively, formed on the housing 15 of the control valve 14. Perforated bases of the members 26 and 27, respectively, engage shoulders 29a and 30 formed, respectively, at junctures of the spool 16 and an extension 32 thereof with a reduced portion 32a. The extension 32, which may be of the same diameter as the spool 16, is spaced from the spool by the reduced portion 32a. The extension 32 has an annular peripheral groove 33 serving as a ball detent pocket and defined by opposed conical surfaces. A sleeve 23 which is mounted in the body 15 of the control valve 14 so as to slide therein, loosely receives the extension 32 of the spool 16 and has at one end a conical surface 35. The body 15 of the control valve 14 has a conical surface 36 which faces the conical surface 35 on the sleeve 34. A plurality of detent balls 37 which are distributed about the extension 32 and may be four in number, lie between the conical surfaces 35 and 36 in engagement therewith. A coil spring 38 surrounds the sleeve 34 and has one end in engagement with the inside of one end of the valve housing 15 and the other end in engagement with an external peripheral flange 39 formed on the sleeve 34. Coil spring 38 urges the conical surface 35 against the balls 37 and the balls 37 against the conical surface 36.

The control valve 14 may be shifted manually from the neutral position of FIG. 1, by action against an external end portion 40. Control valve 14 is also regulated by a pilot valve 41 which comprises a movable spool 42 and a stationary housing 43 having a central bore 43a in which the spool 42 is slidably mounted. One end of the pilot valve 41 is connected with the end of the cylinder 11 of the hydraulic power unit where the opening 23 is located, by means of a line 44 con nected to the line 21. The other end of pilot valve 41 is connected by a line 45 with an opening 46 in the cylinder 11 at an intermediate or mid-region thereof. A coil spring 47, which engages the inside of the end of the housing 43 where the line 45 is connected, and a deep recess in the facing end of the spool 42, urges the spool to the position of FIG. 1, in which it engages the inside of the end of the housing 43 where the line 44 is connected. The spool 42 has a relatively long reduced portion 48 at an intermediate region. The body 43 of the pilot .valve 41 has three axially spaced annular grooves 49, 50, and 51 formed about the bore 43a in which the spool 42 is mounted. The groove 51 is connected by a passage 53 in the body 43 and a line 54 with drain indicated at 55. The groove 49 is connected to the pump line 18 by a passage 35a in the body 43 and a line 55b leading therefrom. The groove 50 is connected by way of a passage 56 in body 43, a line 57, and an opening 58 in the body of control valve 14, with a chamber 59 in the body. The chamber 59 is formed between sleeve 34, the body 15, flange 39 on sleeve 34, and a shoulder in body 15 facing flange 39. Seals 60 and 61 for the chamber 59 are located in external annular grooves formed in the sleeve 34 and its flange 39.

Let us consider the pressure conditions at various points of the apparatus just described for the position shown in FIG. 1. The pressure in the head end of the cylinder 11 and in line 21 slightly exceeds the pressure in line 22 and in the rod end of the cylinder 11 at all times when the bucket 66 is past center relative to a bucket pivot 68 hereinafter described. The spool 16 of the control valve 14 is in its middle position in which the lines 21 and 22 leading to the ends of the cylinder 11 of the hydraulic power unit 10 are blocked, so that the piston 12 is held against movement in the cylinder 11. The spool 42 of pilot valve 41 is pushed by the spring 47 against the left end of the housing 43, because the lines 44 and 45 leading to the ends of the pilot valve 41 are at the same pressure. With the spool 42 thus positioned, pump pressure exists in the chamber 59 formed between the sleeve 34 and the housing 15, because the chamber 59 is connected with the pump 17 by way of the lines 18 and 55 b passage 35a, groove 49, reduced portion 48 of spool 42, groove 50, passage 56, line 57 and opening 58. Thus, the sleeve 34 is urged to the left as viewed in FIG. 1 against the spring 38 to provide room for the balls 37 between the conical surfaces 35 and 36.

If now the piston 12 is to be moved to the right as viewed in FIG. 1, the spool 16 of control valve 14 is moved to the right so that the line 21 is connected to the pump 17 and the line 22 to the drain 20. As a result, the piston 12 starts its movement to the right. At this time, the piston 12 is to the left as viewed in FIG. 1 of the centrally located opening 46 in the cylinder 11, and the line 22 is thus connected to the line 45 across the space in the cylinder 11 to the right of the piston 12. Since line 22 is at drain pressure, line 45 is also at drain pressure. At the same time, the pump pressure existing in the line 21 also exists in the line 44 by virtue of the connection of the two lines with one another. Thus, the difference between a 1st pressure signal from the pump pressure transmitted by the line 44 to the left end of the pilot valve 41 and the lack of pressure in the drained right end of the pilot valve results in rightward movement of the spool 42 as viewed in FIG. 1.

With such movement, the region of spool 42 to the left of reduced portion 48 disconnects grooves 49 and 50 from one another and so cuts off the chamber 59 from the pump 17; at the same time, the reduced portion 48 connects the grooves 50 and 51 with one another and thereby opens the chamber 59 to the drain 55. So now the chamber 59 in the control valve 14 tends to collapse under the action of the spring 38 which tries to move the conical surfaces 35 and 36 toward one another. Rightward movement of the spool 16 of the control valve 14 wil meantime have brought the groove 33 in the extension 32 on the spool 16 to the balls 37. The conical surfaces 35 and 36 trying to move toward one another by action of the spring 38 with collapse of the chamber 59 as previously described, seat the balls 37 inwardly into the groove 33. The result is a detent action or holding by the balls 37 and groove 33 of the spool 16 of the conical valve 14 in its rightwardly displaced position.

Novelty is felt to reside in sensing the cylinder midstroke position hydraulically, that is strictly by hydraulic connection so as to eliminate mechanical connection back to, and physical actuation by, the bucket itself. More specifically when the piston 12 is moved sufficiently to the right as viewed in FIG. 1, in the cylinder 11, a second pressure signal develops in the midopening 46 formed in the cylinder 11 as the passing piston uncovers the mid-opening. In other words, the line 45 is now connected at pump pressure by virtue of the connection of the lines 45 and 21 with one another across the openings 23 and 46 in the cylinder 11 and the space therein to the left of the piston 12. Therefore, lines 44 and 45 connected to the opposite ends of the pilot valve 41 transmit the same 1st and 2d pressure signals equalling pump pressure or line pressure, and so the spring 47 in the pilot valve 41 returns the spool 42 to the position of FIG. 1. Now the reduced portion 411 of the spool 42 connects the grooves 49 and 511 in the pilot valve 41 with one another, rather than the grooves 50 and 31, and so the chamber 59 in the control valve 14 is connected to the pump 17, rather than to the drain 55.

So as the result of solely hydraulically connecting according to my invention, the sleeve 34 moves to the left as viewed in FIG. 1, moving the conical surfaces 35 and 36 apart in the desired way and allowing the balls 37 to move outwardly out of the groove 33 in the extension 32 of the spool 16 of the control valve 14. Now, the spring 25 returns the spool 16 to the position of FIG. 1 with the result that the control valve 14 blocks the lines 21 and 22, and hydraulic kickout thus stops the piston 12 against further movement to the right as viewed in FIG. 1 in the cylinder 11.

If the piston 12 is to be moved to the right as viewed in FIG. 1 more than just beyond the mid-opening 46 in the cylinder 11, the spool 16 of the control valve 14 is manually held by engagement with the reduced end 40 on the spool 16 in the rightwardly displaced position. Thus, the release of the balls 37 from the groove 33 due to resumption of pump pressure in the chamber 59 is ineffective to return the spool of the control valve 14 to blocking position.

Reference is now made to FIG. 2 for a description of a tractor loader 62 to which the previously described hydraulic apparatus of the present invention may be applied. The tractor loader 62 comprises a body 63, front wheels 64, one being shown, a boom arm 65, and a digging bucket 66. The rear end of the boom arm 65 has a pivot 67 on the tractor body 63. The bucket 66 has a pivot 68 on the front end of the boom arm 65. The boom arm 65 is raised from a lowered full-line position to a raised dotted-line position by means of a by draulic power unit 69 comprising a cylinder 70, a piston (not shown) slidably mounted in the cylinder 70, and a rod 71 secured to the piston and projecting from the cylinder 70. The cylinder 70 has a pivot 72 on a depending bracket 73 secured to the underside of the tractor body 63. The piston rod 71 has a pivotal connection 74 with a mid-region of the boom arm 65.

The hydraulic power unit 11) is used to control the position of the digging bucket 66 with respect to the boom arm 65. The cylinder 11 of the power unit has an end flange 75 which has a pivotal connection 76 with a bracket 77 secured to the tractor body 63. The piston rod 13 of the power unit 10 has a pivotal connection 78 with one end of a lever 79. The other end of the lever 79 has a pivotal connection 80 with a link 61, which in turn has a pivotal connection 82 with the digging bucket 66. A mid-region of the lever 79 has a pivot 83 on the boom arm 65.

When the arm 65 is raised to the dotted line position of FIG. 2, dumping by the bucket 66 will occur when it is swung to the dotted-line position. In the dumping position 0b the bucket 66, the piston 22 of the hydraulic power unit 10 will occupy a position somewhat as shown in FIG. 1; that is, it will be at or near the left end of the cylinder as viewed in FIG. 1.

DUMP POSITION: BUCKET FULL FORWARD The bucket goes full forward with movement of the piston 12 fully retracted to a corresponding dump posi' tion. Movement of the piston 12 to this position will have been brought about by leftward depression as viewed in FIG. 1 of the spool 16 ollthe control valve 14, with the spool 16 in dump position affording connection of the line 22 to the pump 17 and the line 21 to the drain 20.

In digging position, the bucket 66 will be as shown in full lines in FIG. 2. In breakout or carry position, the bucket 66 will be in the lower dotted-line position, dis placed counterclockwise somewhat beyond the fullline position of FIG. 2. It will be observed that when considered with reference to the boom arm 65, the digging bucket 66 will occupy the extreme dumping and carry positions and also the intermediate position which is angularly between the dumping and carry positions.

TWO STAGE: LOAD-CARRY POSITIONS The load position is taken in the first stage.

The piston 12 is so connected and adjusted with respect to the digging bucket 66 through the piston rod 13, the lever 79, and the link 81 that the digging bucket 66 is properly angled with respect to the boom arm 65 Specifically, the bucket 66 will assume digging position in the lowered position of the boom arm when the piston 12 of the power unit 10 has moved in the cylinder 11 to the right as viewed in FIG. 1 just beyond the opening 46 in the cylinder 11. The movement of the piston 12 to the right just beyond the opening 46 switches the line 45 from drain pressure to pump pressure, thus equalizing the pressures in the lines 44 and 45 connected to the ends of the pilot valve 41 and returning the spool 42 thereof to the position of FIG. 1 and applying pump pressure to the chamber 59 in the control valve 14. The result is that the spool 16 of the control valve 14 is returned to the blocking position of FIG. 1 in which blocking of the lines 21 and 22 by the control valve 14 holds the piston 12 against further rightward movement.

The carry position is taken in the second stage. The operator pulls the control valve rightwardly from the neutral or hold position to the fully extended rack back or roll back position, retracting the bucket full back to the carry position.

CARRY POSITION: BUCKET FULL BACK AT ONCE If the digging bucket 66 is to be moved straightway from full forward to the full back, carry position, the spool 16 of the control valve 14 is moved to the right as viewed in FIG. 1 and held there, producing connection of the line 21 and the line 22 to pump pressure and drain pressure, respectively, and producing the full stroke-of-movement of the piston 12 to the right as viewed in FIG. 1. Admittedly in so doing, the piston 12 passes and uncovers the opening 46 at mid-stroke, equalizing pressures in the lines 44 and 45 and causing the spool 42 of the pilot valve 41 to return in the way described to the position as shown in solid lines in FIG. 1.

But the interacting pilot valve 41, the interconnecting pressure line 57, chamber 59, and the spool 16 of the control valve 14 are nevertheless powerless at this time to enable or force the spool 16 to return to blocking position. The reason is that the driver of the loader 62 is as already indicated, forcibly retaining the spool 16 by hand in its rightwardmost position as viewed in FIG. 1, and will continue to do so until the full piston stroke is completed.

BUCKET FORWARD STAGE The drawings do not illustrate the invention applied in the bucket forward stage, which would have definite advantages but would only be employed as a single stage operation. instead, in the actual example illustrated here, the invention is illustrated as applying to pivoting and automatically stopping the bucket in only the bucket back direction.

For the bucket back movement just referred to, it will be evident that relocation of the pressure tap 46 to a different placement along the cylinders length will change the precise position at which the rack back movement is stopped and the bucket is hydraulically locked. And self-evidently, for purposes of the bucket forward single stage movement earlier referred to, relocation of the ball 37 and associated mechanism to the opposite side of the pocket 33 along with an appropriate change in hydraulic connections will change the two stage action so that the full forward movement of the bucket can be the movement automatically interrupted, if desired.

The automatic assumption of digging position by the bucket 66 facilitates operation of the tractor loader 62. The operator of the loader need concern himself only with the lowering of the boom arm 65 to the ground to position the bucket 66 for digging; he is freed of the chore of accurately angling of the bucket 66 with respect to the arm 65, because indexing the bucket to the desired angle is automatically brought about by adjustment of the connection of the lever 79 and link between the bucket 66 and the piston rod 13. The operator is merely required, just before or during lowering of the boom arm 65 by actuation of the power unit 69, to move the spool 16 of the control valve 14 to the right end, as viewed in FIG. 1, a sufficient time and distance for the spool 16 to be held in its rightwardly displaced position by engagement of the ball detents 37 with the groove 33 on the extension 32 of the spool 16. The valve 14 will then kick out of operating position at the required point as described, without further attention of the operator.

What is claimed is:

1. An assembly comprising:

a. a double acting hydraulic power unit comprising a cylinder and a piston mounted therein for longitudinal movement of the cylinder and piston with respect to one another,

b. a neutralizable control valve connecting the ends of the cylinder with a source of hydraulic pressure fluid and drain so as to move the piston in one direction and the other with respect to the cylinder and neutralizable to block both ends of the cylinder by hydraulic lock action to fix the piston and cylinder against relative movement,

c. means for holding the control valve out of the hydraulic-lock, hold setting and maintained in a setting involving connection of one end of the cylinder with the source and the other end with drain, and

(1. further means for causing release of the holding means, and neutralization of the control valve back to hold setting, in response to relative pressures at the said one end and a mid region of the cylinder over which the piston will pass, and including a member responsive to pressure equalization and having hydraulic connection to the fluid inside respectively said mid region and one end of the cylinder.

2. The assembly as specified in claim 1, the further means being a pilot valve having other connections to the source and drain independent of the control valve, the further means operating in response to connection through the pilot valve with drain to enable actuation of the holding means to occur, the further means operating in response to connection through the pilot valve with the source to release the holding means,

the further means being connected by the pilot valve to drain in response to movement of the piston from the said one end of the cylinder toward the said mid region thereon, and being connected to the source in response to movement of the piston from the said one end of the cylinder beyond the said mid region thereof.

3. An assembly comprising:

a. a power unit comprising a cylinder and a piston mounted therein for longitudinal movement of the cylinder and piston with respect to one another,

b. a source of hydraulic fluid under pressure,

. a control valve connected with the source and the ends of the cylinder so as in a first setting to block the flow of hydraulic fluid to and from the cylinder and connect the source with drain, in a second setting to connect one end of the cylinder with the source and the other end with drain, and in a third setting to connect the said one end of the cylinder with drain and the said other end with the source,

(1. spring means for urging the control valve to its first setting,

e. manual means for acting against the spring means to bring the control valve to its second and third settings,

means for holding the control valve in its second setting,

g. a pilot valve for causing release of the holding 7 means, and

h. first and second lines connecting the pilot valve with said one end of the cylinder and a mid region of the cylinder, respectively, for enabling the holding means to be activated during the second setting of the valve,

the first line thereby communicating the pressure of said source to the pilot valve when said one end of the cylinder is connected as aforesaid to said source,

the second line thereby communicating drain pressure to the pilot valve when the piston is between the said one end and the said mid region of the cylinder and the said other end of the cylinder is connected as aforesaid with drain,

the piston being moved with respect to the cylinder toward its said other end by virtue of the connection of its said other end to drain and its said one end with the source, the passage of the piston beyond the mid region of the cylinder at which the said second line is connected shifting the connection of the same from drain to the source and thereby making the pilot valve release the holding means, whereby the spring means returns the valve to its first setting and further movement of the piston with respenct to the cylinder is prefifth line, the pilot valve, and the fourth line and thereby enable the additional spring means to actuate the holding means,

switching of the second line to the source from drain due to movement of the piston beyond the mid region of the cylinder connected to the second line, regulating the pilot valve so as to connect the holding means with the source via the fifth line, the pilot valve, and the third line and thereby enable the source to act against the additional springs means to release the holding means. 

1. An assembly comprising: a. a double acting hydraulic power unit comprising a cylinder and a piston mounted therein for longitudinal movement of the cylinder and piston with respect to one another, b. a neutralizable control valve connecting the ends of the cylinder with a source of hydraulic pressure fluid and drain so as to move the piston in one direction and the other with respect to the cylinder and neutralizable to block both ends of the cylinder by hydraulic lock action to fix the piston and cylinder against relative movement, c. means for holding the control valve out of the hydrauliclock, hold setting and maintained in a setting involving connection of one end of the cylinder with the source and the other end with drain, and d. further means for causing release of the holding means, and neutralization of the control valve back to hold setting, in response to relative pressures at the said one end and a mid region of the cylinder over which the piston will pass, and including a member responsive to pressure equalization and having hydraulic connection to the fluid inside respectively said mid region and one end of the cylinder.
 2. The assembly as specified in claim 1, the further means being a pilot valve having other connections to the source and drain independent of the control valve, the further means operating in response to connection through the pilot valve with drain to enable actuation of the holding means to occur, the further means operating in response to connection through the pilot valve with the source to release the holding means, the further means being connected by the pilot valve to drain in response to movement of the piston from the said one end of the cylinder toward the said mid region thereon, and being connected to the source in response to movement of the piston from the said one end of the cylinder beyond the said mid region thereof.
 3. An assembly comprising: a. a power unit comprising a cylinder and a piston mounted therein for longitudinal movement of the cylinder and piston with respect to one another, b. a source of hydraulic fluid under pressure, c. a control valve connected with the source and the ends of the cylinder so as in a first setting to block the flow of hydraulic fluid to and from the cylinder and connect the source with drain, in a second setting to connect one end of the cylinder with the source and the other end with drain, and in a third setting to connect the said one end of the cylinder with drain and the said other end with the source, d. spring means for urging the control valve to its first setting, e. manual means for acting against the spring means to bring the control valve to its second and third settings, f. means for holding the control valve in its second setting, g. a pilot valve for causing release of the holding means, and h. first and second lines connecting the pilot valve with said one end of the cylinder and a mid region of the cylinder, respectively, for enabling the holding means to be activated during the second setting of the valve, the first line thereby communicating the pressure of said source to the pilot valve when said one end of the cylinder is connected as aforesaid to said source, the second line thereby communicating drain pressure to the pilot valve when the piston is between the said one end and the said mid region of the cylinder and the said other end of the cylinder is connected as aforesaid with drain, the piston being moved with respect to the cylinder toward its said other end by virtue of the connection of its said other end to drain and its said one end with the source, the passage of the piston beyond the mid region of the cylinder at which the said second line is connected shifting the connection of the same from drain to the source and thereby making the pilot valve release the holding means, whereby the Spring means returns the valve to its first setting and further movement of the piston with respect to the cylinder is prevented by blocking of flow of hydraulic fluid to and from the cylinder, an additional spring means for actuating the holding means, third and fourth lines connecting the pilot valve with the source and drain, respectively, independently of the first and second lines, the valve, and the cylinder, and a fifth line connecting the pilot valve with the holding means, the connection of the pilot valve with the source through the first line and with drain through the second line regulating the pilot valve so as to connect the holding means with drain via the fifth line, the pilot valve, and the fourth line and thereby enable the additional spring means to actuate the holding means, switching of the second line to the source from drain due to movement of the piston beyond the mid region of the cylinder connected to the second line, regulating the pilot valve so as to connect the holding means with the source via the fifth line, the pilot valve, and the third line and thereby enable the source to act against the additional springs means to release the holding means. 